User interface devices have been widespread, which enable a user's input motion or operation by manipulating a display screen with the user's finger, stylus, etc. Notably, an increasing number of mobile information devices typified by such as smart phones, tablet computers, electronic books or PDAs (Personal Digital Assistants) have incorporated touch panels serving as a user interface device that receives the user's finger motion as an input signal.
Irrespective of whether a touch panel is incorporated or not, a mobile information device needs to be downsized for a greater mobility. Due to this, in case a mobile information device is equipped with a user interface device in the form of a plurality of physical input keys or buttons, these keys themselves need to be reduced in size, resulting in a risk that the operability of the mobile information device becomes poorer. In contrast, incase a mobile information device is equipped with a user interface device in the form of a touch panel using a plurality of virtual buttons instead of physical buttons, these virtual buttons can be well adjusted in size depending on the functions of applications assigned to these buttons, with a greater capability of improving the operability or maneuverability of the mobile information device.
As for an information device allowing a user to perform an input action by touching a touch panel with the user's finger, it is a key factor for greater maneuverability of the information device to ease a user's action required for enlarging, zooming in or magnifying an image displayed on a display screen of a display of the information device. In particular, for a mobile information device, a display screen of a display is limited in size, and an image such as a text like a newspaper or a magazine, or a picture like a photograph or a map cannot be enlarged to the user's satisfaction. As a result, in many cases, a user is troubled in perceiving information expressed with a string of letters or a pictorial portion of a displayed image. For these reasons, providing the function of enlarging such an image is critical to a user, and therefore, easing the user's action required for image enlargement is also critical to a user.
A so-called pinch-out or outward pinch gesture is known as a user's action or gesture required for image enlargement. The pinch-out gesture is a gesture in which user moves two fingers (e.g., the thumb and the index finger of the right hand of the user) farther apart while touching a touch screen with those two fingers. In contrast, a gesture required for image reduction is a pinch-in or inward pinch gesture in which the user moves two fingers closer together.
For example, Patent Document No. 1 discloses a technique of displaying a partial image of a displayed image in enlargement, by causing a user to perform a particular form of touch to a touch screen with two fingers altogether. In addition, this document also discloses a technique of further enlarging the enlarged partial image by causing the user to move those two fingers farther apart by sliding those two fingers on the touch screen.
In addition, Patent Document No. 2 discloses a technique of recognizing a user's gestures using a multi-point sensing device that performs multi-touch detection (concurrent detection of a plurality of positions of touches). More specifically, when a user performs a gesture event in the form of a given combination of entries, an input action is invoked in association with the gesture event. This document describes the combination of entries (code) in the form of a user's input action provided by the user's gesture of pinching open (or close) two adjacent fingers.